spans

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Span charts are based on species of wood, dimensions of lumber and loads. Since you are building an open air structure with basically no load, you can throw those charts out the window. In your original post a while back, I think you wanted to span 18' with 2x10's as dropped beams on 2 sides with 2x6's on top spanning 16'. The only load on this would be the 2x6's you want on top. I have never seen a span chart for open framing with no load, so I think you will have to rely on common sense. Will a 2x10 span 18'? yes...will it collapse? probably not, but I'd bet money its going to sag. Same if you went to a 2x12. I think I would double up at least using the 2x10's and move the support posts in 2 0r 3ft each, letting the beam overhang the posts on the ends. This would bring the span down to 12 to 14 ft. and with the addition of some angled bracing off of the posts, would even shorten it more. Same for the 2x6's on top...I would think they will sag with that span. You could possibly double up 2x8's and make the oc spacing greater. There is no reason to stay with 16 or 24 oc spacing (no drywall, plywood or decking). Use 3ft spacing (works out with the 18ft run) with staggered bridging to control any bowing.

......just my 2 cents